It has been almost two months since a SpaceX rocket exploded during launch preparation, taking a Facebook internet satellite along with it. The company has been hard at work to track down the cause of the failure, and SpaceX now says the explosion was caused by a helium loading issue. The investigation is not completely over, but SpaceX plans to start test firing rockets again later this month and wants to return to active launches by the end of 2016.

From the start of the investigation, SpaceX was focused on the helium tanks in the upper stage, as that appeared to be the ignition point of the explosion. This is separate from the helium tank issue that famously caused a Falcon 9 rocket to break up in 2015 on its way to the International Space Station. That was due to a faulty strut that was supposed to hold the helium containers in place.

According to the new update, SpaceX believes this “anomaly” (as it’s called because that sounds nicer than giant fireball) was the result of variations in the pressure and temperature of the helium being loaded into the rocket. Founder Elon Musk has called this the most complex failure the company has ever investigated. The explosion occurred rapidly, with just 93 milliseconds between the first sign of a problem and loss of telemetry.

Liquid-fueled rocket engines can be turned on and off as needed, unlike solid rocket boosters that burn continuously until the fuel is exhausted. As liquid fuel is used up, helium is used to maintain tank pressure because it’s an inert gas. That’s why so much liquid helium is being pumped into the rocket. A breached tank can cause serious damage to the rocket, as we saw in the September failure.

SpaceX now says it has been able to replicate the anomaly at its test facilities. The investigation is zeroing in on the three composite overwrapped pressure vessels (COPVs) inside the liquid oxygen tank, which is where the helium is stored. These are lightweight, durable storage modules that look a bit like giant propane tanks. The specific cause of the tank failure is still under investigation, but knowing how to cause it at least allows SpaceX to avoid causing it.

The company plans to start testing of rocket components soon. It hopes to have the tank loading issue completely solved with the help of these tests. At that point, the rocket will be given the green light for more launches late this year.